In the novel Destination Unknown, Mr Leblance is a French official who helps British intelligence agent Mr Jessop in Morocco on the case of the missing scientist Thomas Betterton. From the manner in which he is able to organise and order personnel to search large amounts of ground and arrange for reconnaissance flights, it appears that Leblanc is a senior officer French intellgence or security officer.
When Jessop finally zeroed in on Aristides's research establishment as the place they are looking for, Leblanc believed it was crazy and incredible. There would be great trouble if they were wrong, but nonetheless, he coukld see the logic of Jessop's deductions and agreed that it must be done.